Generally, a writing pen includes at least an ink storage device and an ink inducing device, wherein the ink storage device is used for storing liquid ink or even gelatin ink as an ink source, and the ink inducing device for inducing ink to the nib for the purpose of writing. As consumption of ink during writing, ambient air needs to be allowed into the ink storage device, otherwise vacuum pressure may be generated in the ink storage device and affect outflow of ink. That means, there should be an air communicating passage between the ink storage device and the ambient air to balance air pressure, which is a key point to ensure that the writing pen can provide fluent writing with no ink leakage.
China patent No. 01235948.3 discloses a writing pen with ink storage, which has a dust-proof plate, a labyrinth breathable plug, a nano-permeable plate and an one-way valve to achieve air communication between the ink storage device and ambient air. Such a writing pen can keep air pressure balance, but has some disadvantages such as complicated structure with many parts, too much assembly procedures and high manufacture cost.